FIG. 11 illustrates a-conventional communication system. This communication system includes a base station 101 and n mobile stations 102-1, 102-2, 102-3, . . . ; 102-n, where n is an integer. The base station 101 has m communication channels, where m is an integer smaller than n. Assume that the base station communicates with the n mobile stations via separate wireless communications. In that case, a scheduler is provided in the base station 101 to establish n communications via limited m channels. The scheduler performs scheduling of n communications to m channels.
FIGS. 12A and 12B are diagrams to explain the scheduling. FIG. 12A illustrates a scheme for uniform scheduling that does not take into account the traffic volume while FIG. 12B illustrates a scheme for scheduling that takes into account the traffic volume. It is assumed here that there are six base stations and three channels.
As illustrated in FIG. 12A, a channel (1), a channel (2), and a channel (3) are assigned to the mobile stations 102-1, 102-2, and 102-3 respectively at time t(1) while the channel (1), the channel (2), and the channel (3) are assigned to the mobile stations 102-4, 102-5, and 102-6 respectively at time t(2). The same channel assignment is performed at time t(3) and time t(4) as that performed at time t(1) and time t(2), respectively. In other words, the channels are assigned turn-by-turn irrespective of the amount of data to be transmitted (i.e., traffic). In this system, no channel is assigned to a mobile station to which no data is to be transmitted.
In the case of FIG. 12B it is assumed that the mobile stations 102-2 and 102-4 have large traffic and the other mobile stations have less traffic. In this case, the channels are preferentially assigned to the mobile stations 102-2 and 102-4 at time t(1) and time t(2) while the channels are assigned to the other mobile stations also at time t(3) and time t(4). The scheduling is performed again when there is a change in the traffic as time passes.
Sometimes the quality of the communication line degrades due to noise or the communication interference from another station. One approach to improve the quality is to perform error correction. However, even the error correction does not always result into better quality. The ARQ transmission scheme is used to perform “retransmission” for transmitting a unit of data (i.e., frame) again which cannot be restored by an error correction technique. Selective Repeat (SR), Go Back N (GBN), Stop and Wait (SAW) etc. are the examples of ARQ transmission scheme.
FIG. 13 illustrates the SR scheme in one-to-one communications. In this scheme, only the frames that included error are retransmitted. A receiving station, upon detection of an error in a frame “2”, sends a request to the transmitting station for retransmission of the frame “2”. The receiving station saves, in a buffer, frames “3” to “7”, that do not include an error. Upon re-reception of the frame “2”, which now includes no error, all the frames are passed to an upper layer in an appropriate order.
The receiving station periodically reports the current receiving status to the transmitting station. Concretely, the receiving station returns the latest frame number until which the frames have been received without error. This report is referred to as a “reception notification” and it is indicated by broken line arrows in FIG. 13. The receiving station reports, as a first reception notification, frame number “1”, because, frame “1” is received properly but frame “2” included an error. The receiving station reports, as a second reception notification, frame number “8”, because, all the frames from “1” to “8” have been received properly.
FIG. 14 illustrates the BGN scheme in a one-to-one communication. In this scheme, all the frames transmitted after a frame that includes an error are retransmitted. A receiving station, upon detection of an error in a frame “2”, sends a request to the transmitting station for retransmission of the frame “2” and discards all the frames that are received after the frame “2”. Upon reception of the retransmission request, the transmitting station retransmits the frame “2” as well as all the frames that were transmitted subsequent to the frame “2” until the reception of the retransmission request. The receiving station returns reception notifications in the same manner as in the SR scheme. In other words, the receiving station reports, as a first reception notification, the frame number “1” and reports, as a second reception notification, the frame number “3”.
FIG. 15 illustrates the SAW scheme in a one-to-one communication. In this scheme, the transmitting station transmits a new frame only upon reception from the receiving station of an affirmative acknowledgement (ACK) corresponding to the previous frame and transmits the same frame as the previous frame upon reception of a negative acknowledgement (NAK) from the receiving station. In other words, the NAK is treated as a retransmission request.
A conventional scheduler, which is being standardized by 3GPP (3rd Generation Partnership Project), will now be explained. The 3GPP is one of the standard setting organizations of the 3rd generation mobile communication. FIG. 16 is a schematic of the base station. The base station includes transmission control units 111-1, 111-2, . . . , 111-n, transmission buffers 112-1, 112-2, . . . , 112-n, and a scheduler 113.
The base station includes a transmission control unit and a transmission buffer corresponding to each mobile station (102-1 to 102-n) communicating with the base station 101. The transmission buffer serves to store data passed from the upper layer. Upon reception of the data from the upper layer or reception of a retransmission request or an NAK from a mobile station, the respective transmission control units sends a transmission request and a notification of the amount of transmission data, to the scheduler. The scheduler 113 preferentially assigns channels to mobile stations which are capable of providing a better communication quality with reference to the transmission requests, the amounts of transmission data, and the qualities of communication line notified by the respective transmission control units. The quality of communication line is determined from the average of measurements performed repeated at the respective mobile stations.
Also, upon reception of a reception notification or an ACK notification from a mobile station, each transmission control unit deletes the data corresponding to the frame number specified by the notification together with data corresponding to the frame numbers preceding the specified frame number because the data need no longer be retransmitted.
Incidentally, the ARQ transmission scheme of a prior art transmission control unit may be designed by making use of any one of the usual SR, BGN, and SAW, or alternatively Nch(N channel)-SAW such as HSDPA (High Speed Data Packet Access) of 3GPP in which n SAW units are arranged in parallel. FIG. 17 illustrates an N-ch SAW scheme. In a 1-ch SAW scheme illustrated in FIG. 15, the performance is not good, because, the communication line is not fully used. In the N-ch SAW scheme, therefore, the same communication is divided into N channels each of which is used to perform an individual transmission thread respectively. For example, as shown in FIG. 17, the transmission 1-0 is performed to transmit a frame “0” via the first channel while the transmission 3-0 is performed to transmit the frame “0” via the third channel.
However, the conventional communication system has problems as discussed below.
FIG. 18 is to explain these problems. It is assumed here that the scheme is GBN scheme. Meanwhile, the same explanation is true in the case of the SR, SAW, and the Nch-SAW-ARQ schemes.
It is also assumed that a transmitting station transmits data to a receiving station A. The receiving station A detects the quality of communication line for several times to repeatedly report the average value thereof in a predetermined timing. The receiving station A reports the quality of communication line (see chained line arrows). As there is degradation in the quality of communication line after reception of a frame “A6”, the fact that the quality has degraded is reported with a second notification i.e., “low quality”.
On the other hand, after transmitting a frame “A11”, the transmitting station receives a NAK in association with the frame “A6” followed by transmitting frames. “A6”, “A7” and “A8” via the channel being assigned to the receiving station A. Then, when a “low quality” is notified by the receiving station A, channel assignment to the receiving station A is halted to assign the channel to another receiving station B followed by transmitting frames “B0” and “B1”.
In this manner, since the average value of the quality of communication line is notified from the receiving station A to the transmitting station, there is a time lag before the transmitting station receives the average value. That is, the time slot in which the receiving station A measures the quality of communication line is different from the time slot in which the transmitting station receives the notification, resulting in a differential quality of communication line, and therefore there is a problem that the transmitting station continues assigning a channel to the receiving station A even in the time slot in which the quality of communication line is low.